Generally speaking, high printing precision depends on constantly maintaining media in a printer flat and under tension. In most printers media is moved from a media supply with a platen roller. While media can be kept flat (and under tension) in a forward direction in such printers, reversing the direction is highly likely to cause media buckling, leading to incorrect media edge sensing and print registration. To overcome this issue, a mechanism capable of maintaining media tension in forward and reverse directions is needed.
Standard means of maintaining media tension often suffer some delay in response to reversing media feeding direction, which may result in media buckling. Such a delay may be introduced by the time needed to disengage the gears, or by having some parts (such as a platen roller) directly coupled to a motor, while having the other parts (such as a feed roller) coupled indirectly, and hence requiring more time to be activated. For example, U.S. Pat. No. 8,011,611 by Yoshimaru et al. discloses a recording medium transporting mechanism having a flange member attached on either side of a roll paper, support rollers, and a rewinding roller. U.S. Pat. No. 9,061,522 by Obenshain discloses a reversible printer assembly, which includes printing a label on an end-portion of a label roll and advancing the label roll such that the end-portion passes a cutter mechanism. The assembly has a drive clutch configured to engage with a take-up printer ribbon spool, and a reverse clutch having a return spring. U.S. Pat. No. 5,951,177 by Schanke et al. discloses a bidirectional handheld label printer with a labeling media and ink ribbon drive mechanism. A delay is introduced to pretension the ribbon prior to feeding labeling media and ribbon past the printhead. While these options may be acceptable for printing long labels, they do not leave enough time to compensate for buckling during short-label printing. Moreover, neither of these references discusses an option of using a pair of gears for controlling the media movement in forward and reverse directions, while each gear is paired with a clutch capable of functioning in both drive and slip modes. By avoiding gear disengagement and movement delay, such a combination can prevent damage of printer parts and media buckling.
Therefore, a need exists for a system capable of maintaining sufficient media tension in both forward and reverse media feeding directions. Such a system should be applicable to printing short and long labels, while avoiding mechanisms that may cause jamming and printer part damage.